The present invention relates to an apparatus for providing heads up display imagery to an airman wearing night vision goggles.
Night flying in fixed wing and rotary wing aircraft has become an important and viable tactic in modern warfare, particularly since the advent of the Generation III AN/AVS-6 (ANVIS-6) night vision goggle. These night vision goggles are virtually the only U.S. made night vision goggles designed for use in piloting aircraft, with over 20,000 units ultimately scheduled for manufacture.
The goggle optics are typically focused at infinity to view terrain in front of the aircraft, thus it is almost impossible to read the cockpit instrumentation clearly. Further, the new generation of cockpit lighting is designed to be non-visible to these night vision goggles. Looking under the goggles is difficult since instrumentation may be obscured by the goggle housing and the eyes must be refocused from infinity to a few feet to view cockpit displays and then back to infinity to view the outside scene through the goggle. In addition, diversion of the pilot's attention from the outside world to instrumentation is dangerous in a low level, high speed or close formation flight regime.
A heads-up display mounted on night vision goggles is clearly the best method to provide aircraft information or other sensor information to the pilot without diverting his attention from his flying tasks. The prior art solution to this problem has been to use an instrument panel mounted heads-up display which is fixed in position and must be looked at directly for information to be viewed. Panel mounted heads-up displays are expensive and installation is difficult on smaller aircraft, particularly when retro-fit is required.
Helmet mounted displays also require diverting the operator's attention from the external field-of-view. Mounting the heads-up display on the night version goggles is thus the only practical way to achieve continuous monitoring of aircraft and sensor data while the pilot or other operator observes the external scene.
Several methods have been proposed to inject imagery into the operator's field of view while wearing night vision goggles. One method utilizes a beam or image combiner or splitter in front of one of the night vision goggle's objective lenses so that the heads-up display image is amplified by the night vision goggles image intensifier along with the scene information. However, there are three basic disadvantages of this technique, namely (1) night vision goggles tube failure or blooming on the heads-up display side of the goggle will cause loss of the heads-up display information, (2) the heads-up display information will be the same color as the night vision goggles scene, thus making it difficult to discriminate that information from the scene, and (3) resolution of the heads-up display imagery is limited by the night vision goggles objective lens and amplifier tube.
Another prior art device proposes the use of an image combiner between the eye and the night vision goggles eyepiece or ocular. This technique makes it possible to have the heads-up display information in a color that contrasts with the night vision goggles image and heads-up display information will not be lost if the night vision goggles tube fails. Unfortunately, interposing the image combiner and the associated housing between the operator's eye and the night vision goggles ocular brings the heads-up display eyepiece unacceptably close to the operators eye, precluding the wearing of chemical warfare eye protection, such as the M-43 mask, or standard corrective lenses or spectacles. If the eye relief were increased on the heads-up display side to accommodate eye protection goggles or corrective spectacles, the other side without the heads-up display may exceed its eye relief. However, the greatest limitation of an image combiner interposed between the ocular and the eye is the restriction on the maximum field of view. This is due to constraints on the focusing optics between a fiber optics cable carrying the heads-up display information and the image combiner, and the relationship between image combiner size and field of view. Under these circumstances, the maximum practical field of view is approximately a 14 degree circle. This is too small when compared to the basic 40 degree field of view of the night vision goggles, thus reducing the area available for heads-up display information and cluttering the center of the night vision goggles scene.